Continuous and semi-continuous aluminum billet and ingot casting has become increasingly automated with the availability of computer-controlled casting machines. Conventional EM (electromagnetic) or DC (direct chill) aluminum casting typically involves controllably discharging molten aluminum to one or more semi-continuous casting stations. Each casting station includes a concave bottom block and a surrounding mold. The mold can be either a DC mold or an EM mold. In either case, casting is performed by discharging aluminum onto the bottom block and gradually lowering the bottom block while cooling the mold and the lower portions of the cast aluminum.
Various factors are responsible for the quality of the resulting ingot. Generally, three variables must be closely monitored and controlled to achieve optimum results: molten metal surface level or surface elevation, casting rate, and cooling rate. At one time these parameters were monitored and controlled by skilled human operators. The quality of the resulting ingots or billets varied with the skill and experience of the operators. A very skilled operator was capable of producing excellent and repeatable results. However, quality would vary from shift to shift, and between casting machines. Furthermore, even skilled operators would make occasional mistakes.
In order to eliminate the inconsistencies and unpredictable results of operator-controlled casting, modern casting machines are controlled almost exclusively by computer. Various sensors provide information as casting proceeds so that the three variables noted above can be changed on-the-fly as conditions warrant. Typically, a casting "practice" is programmed into the computer to establish desired parameter profiles for molten metal level, casting rate, and cooling rate. This "practice" is repeated during every cast, so that results are repetitively consistent.
A typical semi-continuous casting system includes a number of individual casting stations. Each casting station has a bottom block and a surrounding mold. The bottom blocks of each casting station are mounted in a row to a common table or support structure, and are dropped at the same rate during casting.
A molten metal distribution launder spans the casting stations. The launder has a number of valved downspouts which are controlled to maintain desired elevations of molten metal within the casting station molds. Some type of sensor, such as a float sensor, is associated with each casting station to determine the actual surface elevation of molten metal within the casting station. Other sensors are provided to monitor other needed parameters, such as temperature and coolant flow rates.
It is convenient to mount molten metal flow control components and associated metal surface elevation sensors from the overhead molten metal distribution launder. This is because of the need for unobstructed access to the metal casting stations from above after casting is completed. The molten metal distribution launder is removed from above the casting stations after casting, taking with it the various components which monitor and control metal flow into the casting stations.
U.S. Pat. No. 4,498,521, to Takeda et al., describes such an apparatus where metal flow components are supported from an overhead metal distribution launder. The Takeda patent describes a float sensor which is supported from a metal distribution launder, as well as a control pin which is positioned by an independently-mounted rotary actuator to control metal flow rates into the underlying casting station. Coordination between the float sensor and the rotary actuator is accomplished by an external controller. In many systems, the external controller is remotely located relative to the casting system. The Takeda system allows control over the surface level of molten metal within the underlying casting stations throughout a cast.
Despite the many advantages of an automated system, complexity can lead to certain disadvantages. One such disadvantage is the unreliability of "high-tech" components--particularly when subjected to the extremely high temperatures of a metal casting system. Another disadvantage is that complex systems often require complex calibration adjustments. In a system such as described by the Takeda patent, it is necessary to calibrate the system so that the float sensor produces an output signal which is meaningful to the external controller. This is accomplished in most systems by an electronic adjustment associated with the float sensor. It could also conceivably be accomplished by altering the programming of the external controller. Either procedure introduces an opportunity for error. Furthermore, both procedures require repeated references by human operator to the sensor itself, located over the casting station, and to the external controller, usually located at some distance from the casting station.
The invention below addresses the disadvantages which have resulted from automation of casting systems. It simplifies the installation, repair, and calibration of molten metal surface elevation control systems while also greatly improving the reliability of such control systems.